Magnetic tunnel transistor with high magnetocurrent

ABSTRACT

A magnetic tunnel transistor (MTT) having a pinned layer that has no antiferromagnetic material in an active area of the sensor. The MTT can include a layer of antiferromagnetic material that is exchange coupled with the pinned layer in an area outside of the active area of me sensor, such as outside the track-width, beyond the stripe height, or both outside the track-width and beyond the stripe height. The pinned layer can also be pinned without any exchange coupling at all. In that case, pinning can be assisted by shape enhanced magnetic anisotropy, by extending the pinned layer beyond the stripe height.

RELATED INVENTIONS

This is a Continuation in Part application of commonly assigned,co-pending U.S. patent application Ser. No. 11/187,665 entitled,MAGNETIC TUNNEL TRANSISTOR WITH HIGH MAGNETOCURRENT AND STRONGERPINNING, filed on Jul. 22, 2005, which is incorporated herein byreference as if fully set forth herein.

FIELD OF THE INVENTION

The present invention relates to magnetic tunnel transistors and moreparticularly to a magnetic tunnel transistor having a pinned layerstructure formed directly on a GaAs base, resulting in improved pinnedlayer pinning and increased magnetocurrent.

BACKGROUND OF THE INVENTION

The heart of a computer's long term memory is an assembly that isreferred to as a magnetic disk drive. The magnetic disk drive includes arotating magnetic disk, write and read heads that are suspended by asuspension arm adjacent to a surface of the rotating magnetic disk andan actuator that swings the suspension arm to place the read and writeheads over selected circular tracks on the rotating disk. The read andwrite heads are directly located on a slider that has an air bearingsurface (ABS). The suspension arm biases the slider into contact withthe surface of the disk when the disk is not rotating but, when the diskrotates, air is swirled by the rotating disk. When the slider rides onthe air bearing, the write and read heads are employed for writingmagnetic impressions to and reading magnetic impressions from therotating disk. The read and write heads are connected to processingcircuitry that operates according to a computer program to implement thewriting and reading functions.

The write head includes a coil layer embedded in first, second and thirdinsulation layers (insolation stack), the insulation stack beingsandwiched between first and second pole piece layers. A gap is formedbetween the first and second pole piece layers by a gap layer at an airbearing surface (ABS) of the write head and the pole piece layers areconnected at a back gap. Current conducted to the coil layer induces amagnetic flux in the pole pieces which causes a magnetic field to fringeout at a write gap at the ABS for the purpose of writing theaforementioned magnetic impressions in tracks on the moving media, suchas in circular tracks on the aforementioned rotating disk.

In recent read head designs a spin valve sensor, also referred to as agiant magnetoresistive (GMR) sensor, has been employed for sensingmagnetic fields from the rotating magnetic disk. The sensor includes anonmagnetic conductive layer, hereinafter referred to as a spacer layer,sandwiched between first and second ferromagnetic layers, hereinafterreferred to as a pinned layer and a free layer. First and second leadsare connected to the spin valve sensor for conducting a sense currenttherethrough. The magnetization of the pinned layer is pinnedperpendicular to the air bearing surface (ABS) and the magnetic momentof the free layer is located parallel to the ABS, but free to rotate inresponse to external magnetic fields. The magnetization of the pinnedlayer is typically pinned by exchange coupling with an antiferromagneticlayer.

The thickness of the spacer layer is chosen to be less than the meanfree path of conduction electrons through the sensor. With thisarrangement, a portion of the conduction electrons is scattered by theinterfaces of the spacer layer with each of the pinned and free layers.When the magnetizations of the pinned and free layers are parallel withrespect to one another, scattering is minimal and when themagnetizations of the pinned and free layer are antiparallel, scatteringis maximized. Changes in scattering alter the resistance of the spinvalve sensor in proportion to cos Θ, where Θ is the angle between themagnetizations of the pinned and free layers. In a read mode theresistance of the spin valve sensor changes proportionally to themagnitudes of the magnetic fields from the rotating disk. When a sensecurrent is conducted through the spin valve sensor, resistance changescause potential changes that are detected and processed as playbacksignals.

When a spin valve sensor employs a single pinned layer it is referred toas a simple spin valve. When a spin valve employs an antiparallel (AP)pinned layer it is referred to as an AP pinned spin valve. An AP pinnedspin valve includes first and second magnetic layers separated by a thinnon-magnetic coupling layer such as Ru. The thickness of the spacerlayer is chosen so as to antiparallel couple the magnetizations of theferromagnetic layers of the pinned layer. A spin valve is also known asa top or bottom spin valve depending upon whether the pinning layer isat the top (formed after the free layer) or at the bottom (before thefree layer).

The spin valve sensor is located between first and second nonmagneticelectrically insulating read gap layers and the first and second readgap layers are located between ferromagnetic first and second shieldlayers. In a merged magnetic head a single ferromagnetic layer functionsas the second shield layer of the read head and as the first pole piecelayer of the write head. In a piggyback head the second shield layer andthe first pole piece layer are separate layers.

Magnetization of the pinned layer is usually fixed by exchange couplingone of the ferromagnetic layers (API) with a layer of antiferromagneticmaterial such as PtMn. While an antiferromagnetic (AFM) material such asPtMn does not in and of itself have a magnetization, when exchangecoupled with a magnetic material, it can strongly pin the magnetizationof the ferromagnetic layer.

The push for ever increased data rate and data capacity has lead a driveto increase the performance and decrease the size of magnetoresistivesensors. Such efforts have lead to an investigation into the developmentof tunnel junction sensor or tunnel valves. A tunnel valve operatesbased on the quantum mechanical tunneling of electrons through a thinelectrically insulating barrier layer. A tunnel valve includes first andsecond magnetic layers separated by a thin, non-magnetic barrier. Theprobability of electrons passing through the barrier layer depends uponthe relative orientations of the magnetic moment of the first and secondmagnetic layers. When the moments are parallel, the probability ofelectrons passing through the barrier is at a maximum, and when themoments are antiparallel, the probability of electrons passing throughthe barrier is at a minimum.

To further increase the signal output generated as a result of reading agiven magnetic signal, some researchers have investigated thepossibility of incorporating tunnel junction sensor technology into atransistor device (tunnel transistor). To date, however, no practicaltunnel transistors have been developed. The failure of such devices hasbeen in large part to the inability to satisfy the needs of a tunnel,valve (such as strong pinned layer pinning) while also meeting the needsof a transistor device (such as the selection of appropriate emitter,base and collector materials).

Therefore, there is a need for a magnetoresistive device that cangreatly increase magnetoresistive output from a given magnetic signal.Such a device would preferably incorporate the magnetoresistiveadvantages of tunnel junction sensors with the large gain advantages oftransistor devices.

SUMMARY OF THE INVENTION

The present invention provides a magnetic tunnel transistor (MTT) havingimproved performance arid robust performance. An MTT according to thepresent invention includes an emitter, base and collector. The baseincludes a free layer, pinned layer and a non-magnetic spacer sandwichedbetween the free and pinned layer. The emitter and base are separated bya thin, electrically insulating barrier layer. The magnetic tunneltransistor has stripe height that is measured from the ABS to the edgeof the free layer furthest from the ABS. The pinned layer can extendsignificantly beyond this stripe height and is exchange coupled to alayer of antiferromagnetic material (AFM) layer in a region outside ofthe active area of the sensor.

The MTT can include a layer of antiferromagnetic material that isexchange coupled with the pinned layer in an area outside of the activearea of the sensor, such as outside the track-width, beyond the stripeheight, or both outside the track-width and beyond the stripe height.The pinned layer can also be pinned without any exchange coupling atall. In that case, pinning can be assisted by shape enhanced magneticanisotropy, by extending the pinned layer beyond the stripe height.

Having an AFM layer in the active region of the sensor significantlydegrades performance by scattering electrons and greatly reducing thenumber of hot electrons passing through the barrier transistor. However,strong pinned layer pinning is needed to maintain robustness.

By exchange coupling the pinned layer with the AFM layer outside of theactive area, the pinned layer can be constructed directly upon asemiconductor substrate such as a GaAs substrate resulting in very largetransistor gain. The AFM layer can be constructed of an electricallyinsulating AFM material such as alpha phase iron oxide to avoid shuntingsense current through the AMF layer. Alternatively, the AFM layer can beformed of an electrically conductive AFM material such as PtMn or IrMn,in which case thin insulation layers can be formed between the AFM layerand adjacent non-pinned layer portions of the MTT and also between theAFM and the adjacent shield.

In addition, the collector can be removed from the gap, by recessing thecollector from the ABS and forming the bottom electrode so that itcontacts the base, and not the collector in this recessed region. Thisgreatly reduces the gap thickness which increases data density when theMTT is used as a sensor in a magnetic data recording device.

These and other features and advantages of the invention will beapparent upon reading of the following detailed description of preferredembodiments taken in conjunction with the Figures in which likereference numerals indicate like elements throughout.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature and advantages of thisinvention, as well as the preferred mode of use, reference should bemade to the following detailed description read in conjunction with theaccompanying drawings which are not to scale.

FIG. 1 is a schematic illustration of a disk drive system in which theinvention might be embodied;

FIG. 2 is an ABS view of a slider illustrating the location of amagnetic head thereon;

FIG. 3; is a cross sectional view, taken from line 3-3 of FIG. 2illustrating a magnetic tunnel transistor according to an embodiment ofthe invention;

FIG. 4 is a cross sectional view of an alternate embodiment of theinvention;

FIG. 5 is a cross sectional view of yet another embodiment of theinvention;

FIG. 6 is a cross sectional view of still another embodiment of theinvention;

FIG. 7 is a cross sectional view of another embodiment of the invention;and

FIG. 8 is an ABS view of another embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description is of the best embodiments presentlycontemplated for carrying out this invention. This description is madefor the purpose of illustrating the general principles of this inventionand is not meant to limit the inventive concepts claimed herein.

Referring now to FIG. 1, there is shown a disk drive 100 embodying thisinvention. As shown in FIG. 1, at least one rotatable magnetic disk 112is supported on a spindle 114 and rotated by a disk drive motor 118. Themagnetic recording on each disk is in the form of annular patterns ofconcentric data tracks (not shown) on the magnetic disk 112.

At least one slider 113 is positioned near the magnetic disk 112, eachslider 113 supporting one or more magnetic head assemblies 121. As themagnetic disk rotates, slider 113 moves radially in and out over thedisk surface 122 so that the magnetic head assembly 121 may accessdifferent tracks of the magnetic disk where desired data are written.Each slider 113 is attached to an actuator arm 119 by way of asuspension 115. The suspension 115 provides a slight spring force whichbiases slider 113 against the disk surface 122. Each actuator arm 119 isattached to an actuator means 127. The actuator means 127 as shown inFIG. 1 may be a voice coil motor (VCM). The VCM comprises a coil movablewithin a fixed magnetic field, the direction and speed of the coilmovements being controlled by the motor current signals supplied bycontroller 129.

During operation of the disk storage system, the rotation of themagnetic disk 112 generates an air bearing between the slider 113 andthe disk surface 122 which exerts an upward force or lift on the slider.The air bearing thus counter-balances the slight spring force ofsuspension 115 and supports slider 113 off and slightly above the disksurface by a small, substantially constant spacing during normaloperation.

The various components of the disk storage system are controlled inoperation by control signals generated by control unit 129, such asaccess control signals and internal clock signals. Typically, thecontrol unit 129 comprises logic control circuits, storage means and amicroprocessor. The control unit 129 generates control signals tocontrol various system operations such as drive motor control signals online 123 and head position and seek control Signals on line 128. Thecontrol signals on line 128 provide the desired current profiles tooptimally move and position slider 113 to the desired data track on disk112. Write and read signals are communicated to and from write and readheads 121 by way of recording channel 125.

With reference to FIG. 2, the orientation of the magnetic head 121 in aslider 113 can be seen in more detail. FIG. 2 is an ABS view of theslider 113, and as can be seen the magnetic head including an inductivewrite head and a read sensor, is located at a trailing edge of theslider. The above description of a typical magnetic disk storage system,and the accompanying illustration of FIG. 1 are for representationpurposes only. It should be apparent that disk storage systems maycontain a large number of disks and actuators, and each actuator maysupport a number of sliders.

With reference now to FIG. 3, a possible embodiment of the inventionincludes a tunnel transistor device 300 sandwiched between first andsecond leads 302, 304. The leads 302, 304 can be constructed of amagnetic material such as NiFe so that they may function as magneticshields as well as electrical leads. The tunnel transistor device 300includes an emitter portion 306, a base portion 308 and a collectorportion 310. Although it can be seen that the collector 310 iselectrically connected with first lead 302, and the emitter 304 iselectrically connected with the second lead 304, the base is actuallyconnected with a third lead that is not shown in FIG. 3 and preferablyextends along a direction out of the page in FIG. 3.

The collector 310 is preferably a GaAs layer having a (001) orientation.As those skilled in the art will appreciate, this is one of the cubiclattice planes. Cubic structures have three axes, for example x, y, z. A(001) orientation is along a plane that is parallel to the x, y planesand cuts the z axis. The collector 310 preferably has a thickness of100-500 Angstroms and can be deposited on the shield 302 by sputtering.

The base 308 is a giant magnetoresistive (GMR) element having a magneticfree layer 312, a pinned layer structure 314 and a non-magnetic,electrically conductive spacer layer 316 sandwiched between the freelayer 312 and pinned layer structure 314. The free layer 312 can beconstructed of Co, CoFe, NiFe or a combination of these layers, and canhave a thickness of about 20-60 Angstroms. Spacer layer 316 can beconstructed of, for example Cu and can have a thickness of 20-40Angstroms. The pinned layer structure 314 can several configurations andis preferably an antiparallel coupled (AP coupled) structure having afirst magnetic layer (AP1) 318 a second magnetic layer (AP2) 320 and anon-magnetic, electrically conductive AP coupling layer 322 sandwichedbetween the AP1 and AP2 layers 318, 320. The AP1 and AP2 layers 318, 320can be constructed of many magnetic materials, and are preferablyconstructed of CoFe, each having a thickness 20-40 Angstroms. The AP1and AP2 layers are more preferably Co₇₀Fe₃₀, Co₅₀Fe₅₀ or a CoFe alloyhaving 70-50 atomic percent Co. CoFe has the advantageous properties ofproviding excellent magnetic GMR performance, and also providing astrong positive magnetostriction, which when combined with compressivestresses in the sensor 300 cause the AP1 and AP2 layers to a desired,strong magnetic anisotropy in a direction perpendicular to the ABS. TheAP coupling layer 322 can be constructed of, for example, Ru and canhave a thickness of 4-8 Angstroms, to provide strong antiparallelcoupling between the AP1 and AP2 layers 318, 320.

The emitter 306 can be constructed of, for example Cu or Al and can havea thickness of 20-40 nm or about 30 nm. The emitter is separated fromthe base by a tunnel barrier layer 324. The barrier layer 324 can beconstructed of several materials including aluminum oxide (Al₂O₃),magnesium oxide (MgO_(x)) or titanium oxide (TiO_(x)). The barrier layer324 preferably has a thickness of 10-30 Angstroms.

With continued reference to FIG. 3, it can be seen that the magnetictunnel transistor (MTT) 300 does not include an antiferromagnetic layer(AFM layer) in the active area of the sensor (ie between the pinnedlayer 314 and the GsAs collector 310) for pinning the magnetic moment ofthe pinned layer 314. A problem experienced with prior art MTT sensorsis that the use of an AFM layer for pinning the pinned layer renderssuch a MTT with a spin valve base essentially useless. This is due tothe loss of hot electrons caused by electron scattering in the AFMlayer. Removal of the AFM layer from the active area of the MTT sensor300 results in a very large increase in magnetocurrent (MC), theresulting increase being several thousand percent over a MTT having anAFM layer.

In order to eliminate the AFM layer and still maintain pinning, it isnecessary to replace the AFM with another suitable pinning mechanism.With reference still to FIG. 3, it can be seen that the sensor 300 has afirst stripe height SH1 as measured from the air bearing surface (ABS).The ABS is located at the surface of the sensor that is closest to themagnetic medium (not shown). Although referred to as an air bearingsurface, this ABS surface could also be referred to as a medium facingsurface, because, as fly heights become ever smaller, the slider couldat some point be considered to be in contact with the medium. This firststripe height is defined by the free layer 312 and preferably otherlayers such as the emitter 306 barrier 324 and spacer 316. It can alsobe seen that the pinned layer 314 extends significantly beyond the firststripe height SH1 to a second stripe height SH2 that is significantlylarger than the first stripe height SH1. This extended pinned layer 314creates a strong shape induced anisotropy in a direction perpendicularto the ABS. This magnetic anisotropy is additive to other pinningmechanisms, including a magnetostriction induced anisotropy andantiparallel coupling. As discussed above, the magnetostriction inducedanisotropy is the result of the strong positive magnetostriction of thepinned layers 318, 320, in combination with compressive stresses In thesensor. In order to ensure effective pinned layer pinning, the stripeheight SH2 of the pinned layer is preferably about 2 times the stripeheight SH1 of the free layer and other layers.

With continued reference to FIG. 3, the pinned layer structure 314 hasan extended portion that is outside of the active area of the sensor.That is to say it extends between from the first stripe height SH1 tothe second stripe height SH2, and is beyond the end of the free layer312, spacer 316, barrier 324 and emitter 306. The active area is withinSH1 and is defined by the stripe height of the free layer 312. Thisextended portion 326 can be exchange coupled with an antiferromagneticlayer (AFM layer) 328. This AFM layer is preferably an electricallyinsulating antiferromagnetic material layer such as Alpha phase ironoxide (Fe₂O₃) or some other electrically insulating antiferromagneticmaterial layer. Making the AFM layer 328 of an electrically insulatingmaterial prevents sense current from being shunted between the leads304, 302 outside of the active area of the sensor. Alternatively, theAFM layer could be constructed of an electrically conductive materialsuch as PtMn, or IrMn. However, in that case, an electrically insulatinglayer of, for example, alumina would be needed at the back stripe heightedge of the spacer 316, free layer 312, barrier 324, and emitter 306.The insulation would also have to extend between the AFM 328 and theadjacent lead 304. Such an insulation layer will be described in greaterdetail below with reference to FIG. 4.

The AFM layer 328 is exchange coupled with the extended portion of thepinned layer structure 314 and more specifically is exchange coupledwith the AP2 layer 320. This exchange coupling strongly pins themagnetic moment 330 of the extended portion of the AP2 layer in adirection perpendicular to the ABS. This in turn pins the entire AP2layer 320. The AP coupling between the AP1 and AP2 layers 318,320 pinsthe moment 332 of the AP layer 318 in a direction opposite to that 330of AP2 320. The AP coupling of the two layers 318, 320,magnetostatically induced anisotropy, and shape induced anisotropy (as aresult of the extended stripe height SH2 of the pinned layer) furtherincrease the pinning of the moments 330, 332. The free layer 312 has amagnetic moment 334 that is biased in a direction parallel with the ABSas indicated, but which is free to rotate in response to a magneticfield from an adjacent magnetic medium. Biasing of the free layer, canbe provided by hard bias layers (not shown), which would be into and outof the plane FIG. 3. The remaining space between the shields 302, 304(that behind the pinned layer 314 and collector 310) can be filled with,an electrically insulating, non-magnetic fill layer 336.

With reference now to FIG. 4, a MTT 400 according to another embodimentof the invention has a structure similar to that described above withreference to FIG. 3, except that it provides the option of using anelectrically conductive AFM layer 402 such as PtMn, or IrMn. The sensor400 includes a stripe height insulation layer 404 formed at the stripeheight edge 401 located at SH1. The stripe height insulation layer islocated between the AFM layer 402 and the spacer 316, free layer 312,barrier 324 and emitter 306. This insulation layer can be constructedof, for example tantalum oxide (TaO_(x)), and can be deposited, forexample by a conformal deposition process such as chemical vapordeposition (CVD) atomic layer deposition (ALD) or some other depositiontechnique. This deposition will deposit the insulation, material to asubstantially uniform thickness across the top of the emitter 306, downacross the first stripe height edge 401 of the sensor, and across thetop of the pinned layer 314. A directionally preferential materialremoval process such as reactive etch (RIE) to remove horizontallydisposed portions of the insulation layer. This will leave only thevertically disposed portion of the insulation layer 404 formed on thefirst stripe height edge 401 of the sensor 400. Thereafter, the ATMlayer 402 can be deposited, and a top insulation layer 406, such asalumina, can be deposited over the top of the AFM layer 402 to insulatethe AFM layer 402 from the top shield 404.

With reference now to FIG. 5, in another embodiment of the invention500, includes a MTT sensor stack 502 that is recessed from the ABS. Thesensor stack 502 is constructed upon a semi-conductor substrate 504,such as a GaAs substrate. The semi-conductor substrate 504 is acollector, and may be referred to herein as a collector 504. The sensorstack is separated from the ABS by first shield 506, first and secondinsulation layers 508, 510 and a magnetic flux guide 512. A secondshield 514 is formed above the sensor stack 502. The first and secondinsulation layers can be constructed of, for example, alumina Al₂O₃ orsome other electrically insulating material. A thin third insulationlayer 516, such as alumina, is disposed between the flux guide 512 andthe sensor stack 502. As with the previously described embodiments, thesensor 500 includes an emitter 518, abase that is made up of a magneticfree layer 520 and a non-magnetic, electrically conductive spacer layer522, and a pinned layer 524. A thin, electrically insulating barrierlayer 521 is sandwiched between the free layer 520 and the emitter 518.

The pinned layer 524 is constructed directly upon the GaAs substrate(collector) 504, and has an extended portion 526 that is exchangecoupled with an AFM layer 528. The AFM layer 328 may be an electricallyconductive AFM material such as Alpha phase Fe₂O₃, or may be anelectrically conductive AFM material such as PtMn, or IrMn, in whichcase an insulation layer 530 would be formed in front of the AFM layer,and an insulation layer 532 would be formed above the AFM layer 528 toprevent shunting of current through the AFM layer 528.

As can be seen, the embodiment described above with reference to FIG. 5allows the pinned layer 524 to be formed directly on top of a thick GaAssubstrate 504. As with the previously described embodiments, the shields506, 514 are constructed of electrically conductive material andfunction as electrical leads. The presently described embodimentadvantageously allows the first shield/lead 506 to be in direct contactwith the pinned layer 514, while also allowing the pinned layer 514 tobe constructed directly on top of the collector 504. Constructing thepinned layer 514 directly on top of the collector 504 provides optimalMTT performance. Therefore, this contact between the pinned layer 524and collector 504 can be maintained while virtually eliminatingparasitic resistance through the collector 504.

Although magnetic tunnel transistors discussed above have been describedwith reference to use in a magnetic memory device such as a disk drive,a magnetic tunnel transistor according to the present invention couldalso be used in a magnetic random access memory (MRAM). By way ofexample, a MRAM device can include a series of essentially parallel bitlines and a series of word lines that are parallel with one another andperpendicular to the bit lines. The word lines and bit lines do notactually intersect, but appear to form a grid when viewed from above.Bach word line is connected to each bit line by a magnetic tunneltransistor.

Current flowing through a word and bit line generates a magnetic fieldthat affects the magnetic tunnel junction transistor associated withthat pair of word and bit lines, thereby allowing the magnetic state ofthe magnetic transistor to be switched. The magnetic state of aparticular memory cell (magnetic tunnel transistor) defines a memorystate (off or on).

Additional Possible Embodiments

With reference now to FIG. 6, another possible embodiment of theinvention is described. As those skilled in the art will appreciate, thedistance between the leads 302, 304 determines the total gap thickness.And, since this gap thickness increases data density, it is desirablethat this gap thickness be as small as possible. Because the GaAscollector 310 must be very thick relative to the other layers, it isdesirable that this layer not be included in the total gap thickness ofthe device.

To this end, then, as can be seen in FIG. 6, the collector 310 has beenmoved away from the ABS. The lead 302 has been positioned between thecollector 310 and the ABS, so that the lead is adjacent to the layer 318of the pinned layer 314. A thin insulation layer, such as alumina, hasbeen formed between the collector 310 and the lead 302 to preventcurrent shunting between the lead and the collector 310. The gap fordata reading purposes then becomes the distance between the lead layer302 and the lead layer 304, and does not include the thickness of thecollector 310. As can be seen then this embodiment can be useful forapplications where a further reduced gap thickness is needed.

With reference now to FIG. 7, yet another embodiment is described. Inthe above discussion of FIG. 3, the pinned layer 314 was pinned byexchange coupling layer 320 with an AFM layer 328. However, it is alsopossible to pin the pinned layer 328 without the need for exchangecoupling.

In this case, the pinned layer 314 can be pinned by at least in part bya shape enhanced anisotropy, as well as possibly being further enhancedby a magnetostriction induced anisotropy. As can be seen in FIG. 7, thepinned layer 314 is very long in the direction perpendicular to the airbearing surface (ABS). This long stripe height of the pinned layer 314is much greater than a width (not shown) of the pinned layer 314 aswould be viewed in a plane parallel with the ABS. This results in ashape enhanced magnetic anisotropy oriented in a direction perpendicularto the ABS. In addition, the magnetic layers 332, 330 can be constructedof a material such as CoFe that has a strong positive magnetostriction.This positive magnetostriction, along with the compressive forces in thesensor device that result from lapping operations during manufacture,cause such a strong magnetic anisotropy perpendicular to the ABS, thatpinning can be maintained even without any exchange coupling with an AFMlayer.

FIG. 8 illustrates yet another embodiment of the invention. FIG. 8, is aview of a tunnel transistor device as viewed from the air bearingsurface (ABS). As can be seen, the layers 316, 312, 324, 306 have awidth that defines a track width TW, which defines an active area of thedevice. The pinned layer structure 314, however, extends significantlybeyond the track width TW (ie. beyond the sides of the layers 316, 312,324, 306). Layers of antiferromagnetic material 802, 804 are exchangecoupled with the layer 320 in areas outside of the track width. As withsome of the above described embodiments these AFM layers 802, 804 can beconstructed of an electrically insulating non-magnetic material. Thisstrongly pins the magnetization 330 of the layer 320. Therefore, as canbe seen, although FIG. 3 showed the pinned layer being exchange coupledwith the AFM layer 328 in a region beyond the stripe height SH1, thepinned layer can be more generally described as being exchange coupledin an area outside of an active area of the sensor, such as at eitherside as shown in FIG. 8. The pinned layer 314 could even be pinned inboth in areas beyond the stripe height SH1 (FIG. 3) as well as in areasoutside of the track-width TW (FIG. 8).

While various embodiments have been described above, it should beunderstood that they have been presented by way of example only, and notlimitation. Other embodiments falling within the scope of the inventionmay also become apparent to those skilled in the art. Thus, the breadthand scope of the invention should not be limited by any of theabove-described exemplary embodiments, but should be defined only inaccordance with the following claims and their equivalents.

1. A magnetic tunnel transistor (MTT) having an air bearing surface(ABS), comprising: an emitter; a collector; a base sandwiched betweenthe emitter and the collector, the base comprising a magnetic freelayer, a magnetic pinned layer and a non-magnetic spacer layersandwiched between the free layer and the pinned layer, the free layerhaving a dimension that defines an active area of the sensor, and thepinned layer having portion that extends beyond the active area of thesensor; and a layer of antiferromagnetic material contacting theextended portion of the pinned layer.
 2. A magnetic tunnel transistor asin claim 1 wherein the free layer defines a track-width, and wherein thepinned layer extends beyond the track-width, and contacts the layer ofantiferromagnetic material in an area outside of the track-width.
 3. Amagnetic tunnel transistor as in claim 1 wherein the free layer definesa track width measured parallel with the ABS and defines a stripe heightmeasured from the ABS in a direction perpendicular to the ABS, andwherein the pinned layer extends beyond both the stripe height andtrack-width and contacts the layer of antiferromagnetic material in theregions beyond the track-width and beyond the stripe height.
 4. Amagnetic tunnel transistor as in claim 1 wherein the layer ofantiferromagnetic material comprises an electrically insulatingnon-magnetic material.
 5. A magnetic tunnel transistor as in claim 1wherein the layer of antiferromagnetic material comprises Fe₂O₃.
 6. Amagnetic tunnel transistor as in claim 1 wherein the layer ofantiferromagnetic material comprises Fe₂O₃.
 7. A magnetic tunneltransistor as in claim 1 wherein the collector comprises GaAs.
 8. Amagnetic tunnel transistor (MTT) having an air bearing surface (ABS),comprising: an emitter; a collector; and a base sandwiched between theemitter and the collector, the base comprising a magnetic free layer, amagnetic pinned layer and a non-magnetic spacer layer sandwiched betweenthe free layer and the pinned layer, the free layer extending from theair bearing surface by a distance that defines a stripe height, and thepinned layer extending beyond the stripe height, the the pinned layerstructure having an elongated shape in the stripe height direction thatresults in a shape enhanced magnetic anisotropy in a directionperpendicular to the air bearing surface that assists in maintainingmagnetic pinning of the pinned layer without exchange coupling withexchange coupling with a layer of antiferromagnetic material.
 9. Amagnetic tunnel transistor as in claim 8 wherein the pinned layerfurther comprises first and second magnetic layers separated by anon-magnetic antiparallel coupling layer sandwiched therebetween.
 10. Amagnetic tunnel transistor as in claim 8 wherein the pinned layerfurther comprises first and second magnetic layers separated by anon-magnetic antiparallel coupling layer sandwiched therebetween, andwherein the first and second magnetic layers have a positivemagnetostriction that combines with compressive stresses in the magnetictunnel transistor to create a magnetic anisotropy perpendicular to theair bearing surface that further assists pinning.
 11. A magnetic tunneltransistor as in claim 8 wherein the pinned layer further comprisesfirst and second magnetic layers each comprising CoFe separated by anon-magnetic antiparallel coupling layer comprising Ru sandwichedtherebetween.
 12. A magnetic tunnel transistor as in claim 8 wherein thecollector comprises GaAs.
 13. A magnetic tunnel transistor as in claim 8wherein the collector comprises GaAs having a (001) crystal structure.14. A magnetic tunnel transistor as in claim 8 further comprising alayer of electrically insulating, non-magnetic material covering thepinned layer in the region beyond the stripe height.
 15. A magnetictunnel transistor (MTT) having an air bearing surface (ABS), comprising:an emitter; a collector; a base sandwiched between the emitter andcollector, the base comprising a magnetic free layer, a magnetic pinnedlayer and a non-magnetic spacer layer sandwiched between the free layerand the pinned layer, the free layer having a dimension measured fromthe ABS to define a stripe height, and the pinned layer having a portionextending beyond the stripe height defined by the free layer; and alayer of antiferromagnetic material contacting the extended portion ofthe pinned layer; and wherein the collector is recessed from the ABS.16. A magnetic tunnel transistor as in claim 15 further comprising anelectrically conductive lead, contacting the base in a region betweenthe collector and the air bearing surface.
 17. An magnetic tunneltransistor as in claim 15 further comprising an electrically conductivelead, contacting the base in a region between the collector and the airbearing surface, and a non-magnetic, electrically insulating materialseparating the collector from the lead in the region between thecollector and the air bearing surface.
 18. A magnetic tunnel transistoras in claim 15 wherein the layer of antiferromagnetic material comprisesan electrically insulating antiferromagnetic material.
 19. A magnetictunnel transistor as in claim 15 wherein the layer of antiferromagneticmaterial comprises Fe₂O₃.
 20. A magnetic tunnel transistor as in claim15 wherein the layer of antiferromagnetic material comprises alpha phaseFe₂O₃.